Fire alarm system



Patented May 29, 1945 PATENT ortica FIRE ALARM SYSTEM application teneis, 1944, serial No. 540,916

lll Claims.

This invention relates to automatic firev alarm systems and more particularly to improvements in fire alarm systems of the thermocouple type.

My invention has been particularly adapted l and applied for detecting fires on airplanes, and

it is in that connection that I herein describe my invention, but no unnecessary limitation of my invention to this particular application is intended.

In airplane applications there is a tendency for the re alarm systems 'to fail because of open circuits caused by the vibration of the plane. Also, in applications where the thermocouples are exposed to the weather, such as in detecting engine lres, the insulation resistance between the thermopile circuit and the casing of the thermopile will tend to be reduced, giving rise to leakage points which result in low resistance shunts across the respective thermocouples and, again, in possible failure of operation of the re alarm system. In practice, especiallyin damp weather conditions, there may occur a substantial reduction in insulation'resistance of the thermopile at many points. However, these leakage points are not detrimental until their effective leakage resistance reaches a prescribed threshold value below which the respective thermocouples shunted by these leakage points can no longer be relied upon for dependably initiating an alarm. It is accordingly apparent that it is not only important that re alarm systems in applications of the character mentioned shall have provision by which they may be readily tested fo'r open circuits and leakage resistances but that, as to the latter, the test indications shall be dependent on the magnitude of the leakage resistance so that indications of operability and inoperability on test shall be reliable and shall correspond with actual conditions in operation.

had to the accompanying drawing,

It is an object of my invention to provide an 'I improved circuit for re yalarm systems of the character mentioned and particularlyan improved such system for airplanes having provision by which it may be readily tested for operability.

It is another object to provide improved means for testing such alarm systems for operability which is simple and highly reliable.

It is another object to provide improved means for testing such fire alarm systems which will in one test operation give reliable indications both as to the presence of open circuits and effective leakage resistances. l

It is another object to provide a test means for such re alarm systems which will indicate operability or inoperability according to whether the leakage resistance in the thermopile is above prescribed threshold-value, and further to provide such test means which is adapted to indicate operability only when that threshold value is above the minimum value of leakage resistance permissible in actual operation by a p reor below a .scribed minimum margin of safety.

It is a further object to provide improved means fortesting thermocouple-type fire alarm systems wherein the threshold values of leakage resis* ance on test are above permissible minimum values of those resistances by a uniform margin of safety for all points of leakage along the thermopile.

It is a further object to test thermocouple-type re alarm systems for operability by test current supplied by the potential source which is provided or actuating the alarm of the system.

It is a further object to fulll the aforestated objectives in alarm systems wherein supplementary current is supplied to the relay from thel alarm circuit of the system when the contacts of the relay are closed.

These and other objects of my invention will more fully appear from the following description and the appended claims.

In the description of my invention reference is of which:

Figure i' is a diagrammatic view of the circuits and mechanism showing the .application of my invention to an airplane;

Figure 2 is a schematic circuit diagram of the alarm circuit of Figure 1; andl Figure 3 is a schematic circuit diagram of a second embodiment of my invention.

In Figure 1 there is shown a fire alarm system for detecting tires on an airplane I0 (ractionally shown). The main components of this fire alarm system comprise a thermopile Il', a. sensitive meter relay l2, a battery i3 and an alarm device which may be a signal light I4. Purely by way of example, the thermopile Il is mounted on the fire wall I5 behind the airplane engine I6 (diagrammatically shown) so that it will detect engine oil iires, but it will be understood that the thermopile may be located in any other desired place. This thermopile comprises a number of individual thermocouple units l1 connected l in` the thermocouple units and asbestos in the cables. This casing is clamped against the re wall l5 @by suitable brackets (not shown) and connection to the thermopile is made by way of a connector block extending through the flre wall. For further detailed description of a form of thermopile which is satisfactory for use with the present invention, reference may be had to the pending application of Frederick G. Kelly, Serial No. 442,759, entitled Thermopile, and having a common assignee with the present application.

The meter relay it which is herein preferably ,employed is of the moving magnet type. Such meter relay has for its armature a permanent magnet 2i mounted on a shaft 922, and has one or more ileld .coils t8 for producing a magnetic field to exert a torque iniluence on the armature. Preferably, the magnet 2l comprises a hai`- shaped piece of Alnico and there are employedv two held coils mounted on diametrically opposite sides of the shaft 22. Surrounding the coils and magnet is a cylindrical magnetic shield 26, fractionally shown. The magnet is torsionally biased by a spiral spring 2li so that it normally occupies a position at angles to the asis oi the coils and will be propelled from that position towards the coll axis when the coils are energized.

close positively once the arms 26 and 28 momentarily contact each other, and that the relay switch will stay closed under heavy vibration so long as a normal operating level of current is supplied tothe coils of the relay, supplementary current is supplied to the relay coils from the battery I3 upon closure of the relay switch. The circuit of this supplementary current is from the positive terminal of the battery I3 through the contact arms 26 and 28, a resistor 38, coils 2t, and the lead 29 to the ground point 3l and the negative terminal of the battery. The resistor 38 is provided for the purpose of limiting the supplementary current to such appropriate level that when the thermal current of the thermopile falls to a predetermined level, the spring 2t will overcome the torqueexerted by the coils Carried by the shaft 23 atright angles thereto is a contact arm il@ or' a switch 2l which, as theI armature is propelled by the coils, impinges against a semi-stationary contact arm 28 mounted at right angles to the nrst contact arm. These contact arms are preferably made of thin horrible wire so that they will flex slightly under inrluence of the propellingr torque of the armature to sive a rolling contact as the arras impinge against one another, such manner of engagernerit of the arms being important in sensitive meter relays having small operating power for preventing sticking of the contacts.

As shown in Figure l. one end of the relay coils Y 23 is connected by lead 2Q to ground-i. e.. to the plane structure at point Srlas ls also one end oi the thermopile through lead S3 and the pole mem- Iber il@ and contact point ill ci' a manually-operable switch The other ends of the coils and thermopile are interconnected by lead 29a. Thus. there is formed a closed circuit which includes the thermopile and relay coils in series. This circuit-which is referred to as in the schematic diagram ci llgure 2-is herein termed the control circuit of the alarm system.

The negative terminal of the battery it is connected to the plane structure atthe point 34 and is connected to one side of the lamp lil, and the ncsitive terminal of the battery is connected through the relay switch 2l and a resistor 3G to the other side of the lamp. yThis circuit. which is referred to as Elin Figure 2. is herein termed the alarm circuit of the system. Upon one or more oi the' tliermocou'oles` of the thermopile Il being exposed to a sudden dange'ously-high temperature. there will be generated a thermal voltage which will energize the coils 2t of the relay and cause the switch 2l of the relay to close. When this relay switch is closed. current is supplied to the lamp it from the battery le and a signal light or alarm is given. The function of the resistor is to limit the initial high rush of current through the lamp. when the lamp is cold vand its resistance conseauently is low. so as to on the magnet 2i and will cause the relay switch t0 open. For a typical battery potential of 14 volts, an appropriate value for resistor 38 is 10,000

ohms.

In thermocouple-type re alarm systems the relay is required to close in response to very little power input in view of the small voltage which thermocouples are capable of generating and the practical need for an alarm to be given when only a single thermocouple is exposed to a sudden dangerously-high temperature. The thermal voltage of thermocouples varies with the temperature differential between their hot and cold junctions. Typically, the maximum thermal voltage which a thermocouple can generate is f the order of l0 mv., but this maximum voltage is obtained only when the thermocouple is exposed to such intense heat that it is just short of being destroyed. As a'practical matter, the alarm system must be designed to sound an alarm 'when a much lower value of thermal voltage is generated in the thermopile. By way of preferred example,

the coils 223 may have approximately 3 ohms and he adapted to operate the relay contacts in response to 3 ma. of current; the thermopile may comprise twenty-seven thermocouples in series or which each may have approximately .t7 ohm,

voltage from the thermopile-which is a pre-r scribed suitaicie value for a clependably-operatingv alarm system.

When leakage points develop between the thermccouple circuit and the casing it) ci the thermopile, shunt circuits are created across the respective thermocouples by way of the ground connection of the casing i9 to the plane structure. indicated by 39 in Figure 2. As a result.' part of the thermal voltage of the thermocouples is depleted through these shunt circuits and a greater thermal voltage is therefore required in order to initiate an alarm. rlhe very minimum values of resistance which these leakage points may have while yet maintaining an operable system may be prescribed on the basis of the maximum thermal voltage which the respective thermocouples can generate. Considering, for instance, this maximum voltage to be 40 mv., and that one end of the thermopile i-s grounded as above described, a leakage point between the first and second thermocouples at the grounded end of the thermopile may have a resistance 'as low as a fraction of .i ohm; on the other hand, the minimum permissible resistances of leakage points located between higher-numbered thermocouples are successively higher, the minimum value at the extreme other end of thethermopile for a yet operable system being approximately .'Z ohm.

On testing a re alarm system, it is desired that operability shall not be indicated for leakage resistances in the thermopile as low as the above y prescribed minimum values but, only for leakage resistances which are above a threshold value that is higher than these minimum values by a prescribed minimum margin of safety. In other words, it is important that operability shall not be indicated by the presence of leakage points having the minimum permissible values of leakage resistance, and it is equally important that inoperability shall not be indicated by leakage resistances which are substantially greater than the permissible minimum values by a suitable margin of safety. It is found that thermopiles of ire alarm systems may be tested reliably in one operation for both continuity and leakage resistance and that, as to the latter, the test can be made to indicate operability and inoperability according to whether the effective leakage resistance is above or below the desired threshold abovementioned-which threshold is a minimum value of leakage resistance above which the system is considered dependable in actual operation. To obtain this correspondence on test with conditions in actual operation, it is found to be a basic requirement that the control circuit 35 be connected to ground through substantially negligible resistance or, at most. through a resistance not substantially greater than that of the thermopile. It will be observed that this limitation prescribes that the resistor 38 of the supplementary-current circuit shall be connected between the control and alarm circuits from the side of the coils 23 which is opposite to ground, this being the location of the resistor as above described.

By the present invention, the alarm system is tested simply by supplying current of prede-4 termined value from the battery I3 through the thermopile I I and coils 23 in series, with the coils being connected between the thermopile and ground. This is accomplished by acircuit 4@ serially including a current-limiting resistor 4I and connected from the ungrounded side of the battery I3 to a contact point 42 of the manuallyoperable switch 32. When the pole member of this switch 3 2 is thrown from its normal or run position-the position it occupies in the iigures-to test position wherein it makes connection with the contact point 42, the battery I3 and resistor 4I- are serially interposed in the control circuit between the thermopile and ground and test current is supplied through the thermopile and coils 23 in series. i The value of this test current is chosen so that when the effective leakage resistances are above and below the desired threshold value abovementioned, the system will test operable and inoperable respectively. After many experiments and field tests, it has been found that a preferred threshold value oi leakage resistance for the present illustrative example of my invention is between 1 and 2 ohms; also, it is found that upon supplying a test current of the order of 11 ma. the threshold value on test falls within this desired range. For instance, with this value of test current, the system will test operable and inoperable respectively when the leakage resistance is above and below approximately 1.8 ohms for leakage points at the end of the thermopile next to ground and 1.2 ohms for leakage points at the other end of the thermopile. For a battery potential of 14 volts, the resistor 4I has therefore approximately 1250 ohms.

In Figure 3 I provide a modied test circuit which is differentially-sensitive to the magnitude of leakage resistances at different points along the thermopile in the same way that the system is sensitive in actual operation to the magnitude of such differently-located points of leakageresistance. 'Ihis test circuit includes a resistor 43 connected from the pole 30 of switch 32 to ground. When the switch 32 is in run position, resistor 43 is shorted and has no effect in the circuit; but when the switch 32 is thrown to test position, this resistor is connected serially with a resistor 44, corresponding. to the resistor 4I of Figure 2, to form a potentiometer. The function of this potentiometer is to provide the thermopile and coil in series with a source of current supply having a low internal resistance-preferably a resistance less vthan that of the thermopile in the particular cited example. By way of illustration, the resistor 43 may have .5 ohm, and approximately ma. current may be drawn from the battery I3 through the resistor 44, in which case the resistor 44 will have approximately 175 ohms. Under these conditions, the threshold valueiof -leakage resistance at the end of the thermopile nearest to ground below and above which the system will test as being inoperable and operable respectively is approximately .Bohm and that for leakage points at the other end of the thermopile is approximately .9 ohm. The minimum permissible leakage resistancesat these points, gas aforementioned, are a fraction of .l ohm and approximately .7 ohm respectively. Accordingly, with this modified test circuit the threshold 14esistances on test are above the respective prmissible minimum values by approximately the same margin of safety irrespective of the location of the leakage points along the thermopile, this margin being in the particular instance approximately .2 ohm. Of course, other uniform margins of safety may be prescribed by choosing suitably different values of resistances 43 and 44. Since this test circuit indicates by a substantially uniform margin of safety, a wholly reliable indication is always obtained and more effective use of the alarm system is assured. f

While I have herein shown and described my invention in terms of several specic embodiments, it will be understood that these embodiments are illustrative and not limitative of my invention and are subject to changes and modifications within the range of engineering skill without departurefrom the scope of.my invention, which l endeavor to express according to the following claims.

I claim:

1. In a firm alarm system including a thermopile. a relay controlled by said thermopile and an alarm circuit controlled by said relay and including a source of potential, said sourceoi potential having connection to ground: the combination of a control circuit serially including said thermopile and a coil of said relay; and means for testing said system for operability including a switch in said control circuit for connecting said source of potential to said thermopile and coil in series, with said coil being connected between the thermopile and ground.

2. In a re alarm system including a thermopile, a relay controlled by said thermopile and an alarm circuit controlled by said relay and including a source of potential, said source of potential having one side thereof connected to ground: the combination of a control circuit serially including said thermopile and a coil of said relay, said coil being connected at one end thereof to ground; and means for testing said system for operability including switch means for connecting said source of potential to said thermopile and coil in series, with said coil being connected between said thermopile and ground.

3. In a re alarm system including a thermopile, a relay controlled by said thermopile and an alarm circuit controlled by said relay and including a source of potential, said source of potential having connection to ground: the combination of a control circuit serially including said thermopile and a coil of said relay; support means for said thermopile having connection to ground; and means for testing said system for operability, including testing said thermopile in one operation both for continuity and for leakage to ground, comprising switch means serially included in said control circuit and operable to open said circuit and connect said thermopile and coil to said potential source in series, with said coil being connected between the thermopile and ground.

4. In a fire alarm system including a thermopile, a relay controlled by said thermopile and an alarm circuit controlled by said relay and including a source of potential, said source of potential having one side thereof connected to ground: the combination of a control circuit serially including said thermopile anda coil of said relay, said coil being connected at one end thereof to ground through a resistance at most not substantially greater than the resistance of said coil and thermopile in series; support means for said thermopile having connection to ground; and means for testing said system for operability, including testing said thermopile for continuity and for leakage resistance to ground, comprising means for supplying current from'said source of potential through said thermopile and coil in series, with said coil being connected between said thermopile and ground.

5. In a nre alarm system including a thermopile, a relay controlled by said thermopile and an alarm circuit controlled by said relay and including a source of potential, said source of potential having one side thereof connected to ground: the combination of a control circuit serially including said thermopile and a coil of said relay, said coil being connected at one end thereof to ground, the resistance of said connection being not greater than the resistance of said coil and thermopile. in series; a metal casing insulatedly carrying said thermopile and having connection to ground; and means for testing said system for operability, including testing said thermopile for continuity and for leakage resistance to ground, comprising circuit means including said source of potential for supplying current, within a predetermined range of magnitude, through said thermopile and coil, with said coil being connected between said thermopile and ground.

6. In a fire alarm system including a thermopile, a relay controlled by said thermopile and an alarm circuit controlled by said relay and including a source of potential, said source of potential having one side thereof connected to ground: the combination of a control circuit serially including said thermopile and a coil of said relay, one end of said coil having connection to ground and the resistance oi' said connection being not greater than the resistance of said coil and thermopile in series; a support means-for saiclvthermopile4 having connection to ground: and means :for testing said system for operability,

including testing said thermopile for c wntinuity and for leakage resistance to ground, comprising a current-limiting resistor of predetermined value and means for connecting said potential source through said resistor to said thermopile and coil in series, the said coil being connected between the thermopile and ground.

7. In a fire alarm system including a thermopile, a. relay controlled by said thermopile and an alarm circuit controlled by said relay and including a source of potential, said source of potential having one side thereof connected to ground: the combination of a control circuit serially including said thermopile and a coil of said relay, one end of said coil having permanent connection to ground; support means for Asaid thermopile having connection to ground; a test circuit connected to said source of potential and serially including a current-limiting resistor; and a single-pole two-position switch selectively operable for closing said control circuit or for opening said control circuit and serially connecting said thermopile and coil in said test circuit, with said coil being connected between the thermopile and ground.

8. In a fire alarm system including a thermopile, a relay controlled Aby said thermopile and an alarm circuit controlled by said relay and including a source of potential, said source of potential having one side thereof connected to ground: the combination of a support for said thermopile having connection to ground; a control circuit serially including said thermopile and a coil of said relay, said circuit being connected vto ground at a junction thereof between said coil ground, said means being adapted to supply a including a source of potential, said source of potential having one side thereof connected to groundithe vcombination of a support for said thermopile having connection to ground; a control circuit serially including said thermopile and a coil of said relay, said circuit being connected to ground at one end of said coil, said thermopile comprising a plurality of serially connected thermocouples each capable of generating a thermal voltage suillcient to actuate said relay provided the leakage resistances of the thermopile v to ground across the respective thermocouples are above respective threshold values; and means including said source of potential for supplying test current through said thermopile and coil, with the coil being connected between the thermopile and ground, the value of said test current being predeterminately selected so that actuation of said relay will occur on test only when the leakage resistance of said thermopile to ground is above the maximum of said threshold values.

10. In a re alarm system including a thermopile, a relay including a coil and a pair of coneccesso' tacts, and en alarm circuit serially including said contacts, an alarm device, and c, source oi potential, said source having connection to ground: the combination of e control circuit serially including said coil end thermoplle. one side of sold coil having connection to ground; Vcircuit moons for supplying Supplementary current from seid alarm circuit through said coil to ground upon closure of seid contacts, seid circuit means including o current-limiting resistance connected `oetween. the side of seid coil opposite to ground and seid alarm circuit; and means fortestine seid system for operability, including testing said thermopile ffor continuity ond leakage resistance to ground, comprising means selectively controllable for supplying current from said potentiell source through said thermopiie and coil in series, with seid coil being connected between the thermopile and ground.

11. ln an alarm system for deteotinf,7 lires on airplanes comprising s, thermocouple provided with e, metallic cosine and edepd for mounting on the structure oi en eirplene, e, reloy controlled hy seid thermopile. ond on alerin circuit including e buttery listing one side thereof connected 'to said plone structure: the combination of o control circuit seriolly including seid thermopile and e, coil of seid relay, seid control circuit being connected to seid pleno structure ot one end of seid coil; und ineens for testing; seid system, for operobility, including testine seid theoplle for continuity end leakage resistance to scid plone structure, comprising means for supplying current from sold buttery through seid thermoolle and coil in series, with seid coil heine connected between the thermopile und the plone structure.

l2 in en alarm system iordetectingl rires on airplanes, comprlsinr e themoioile helling c metelllc cosine on und grounded to the structure or the airplane, ereley comprising n coil and s pelr of contacts, end en cierro circuit including c buttery having lone side thereof grounded to seid plone structure end further including seid contacts end on clarin device, seid elorru device being connected ltwcen the grounded side of sold lcottery end sold conteetsi trie combination oi e. control circuit serielly including seid thermopile end coil, seid circuit being grounded et one side ol7 seid coil to seid. clone structure; meons'ior supplying supplementary 4current to seid coil upon closure of seid contacts', including e resistor connecting tne other side of seid coll to o. ooint in seid alarm circuit between said contocts ond elorm device; und circuit means for supplying test' current from said battery through seid thermopile and coll in series, with the coil being connected vbetween the thermopile und plone structure, said circuit means includinse resistor for limiting the test current so that the system will operate on test only when the leekege resistance of the thermopile to the plone structure is greater then that which would cause o failure of the system to onerete in noel use.

13. In an alarm system for detecting rcs on airplanes, comprising a thermopile hsving e. metallic casing mounted on and grounded to the structure of the airplene, a, relay comprising a coil and a pair of contacts, and an alarm circuit inciuding said contacts, an alarm device und e. battery grounded to said plane structure, seid alarm 1 device being connected between the grounded side of seid battery and said contacts: the combination of e, control circuit serielly 1ncluding scid thermopile and coil and having ground connection at one sideA of seid coil to said plane structure; and means for supplying supplementuxy current to 'seid coil upon closure of said contacts comprising e. circuit connecting said coil in parallel relation to seid alarm device, said circuit including e. current-limiting resistor connected between the other side ci sold coil and seid alarm circuit.

le. In e fire alarm system including e thermopile, o relay controlled by seid thermopile and en clerm circuit controlled by seid relay and including e source oi potentiel, seid source ol potential having connection to ground: the combinotion of o, control circuit serially including7 said thermopile ond e coil of sold relay; und circuit means for supplying test current from seid potentiel source through seid thermopile eind coil in series, with the coil loelnc connected between the ther-moelle and ground, whereby to test the system for onereloility, scid circuit ineens inoluding o potentiometer for limiting the test current through seid thermopile and coil.

i5. The comloinotion set forth in cl is wherein seid potentiometer provides e. source or" current suploly to seid thermoplle end coil no1/ing on internal resistonce sulistontiolly less than the resistentie oi seid thermooile.

iS. The combination sei forth in cleim lo wherein sold potentiometer provides e source of potentiel, for supplying current to seid thermopile and coil in series, honing e voltage euuel epproxiinoteiy to the generated thermos/citarse oi the respective thcrmocouples or solo thermoplle.

i7. In o :lire olerm system including o thermoplle, o relay controlled Toy sold tliopile und en elsrrn circuit controlled loy seid reley und iucluding o source or ootentiel, seid source oi tentlol having connection to ground: the combinotion of s, control circuit serielly inclue and thermopile end coil, sold circuit losing coected et one side or scid coil to ground; o reslstcnce in seid control circuit between ground end seid thermopiie; e resistonce serially coected to the ungrounded side of seid source oi potentiel; and switch means nei/ing one position for shorting sold first-mentioned resistance endoucther position for connecting the junction we isetween seid mst-mentioned resistce und thermopile to seid second-mentioned ence. 

